The present invention relates generally to electrochemical cells and more particularly to a separator material comprising aramid fibers for use in a lithium/thionyl chloride system.
A fundamental requirement of a battery separator is to separate the positive and negative terminals of a given battery. The separator must be permeable to allow the passage of electrolytic substances so that ion exchange reactions can occur between the terminals. However, in some battery systems, a harsh environment is present within the cells. Many materials which provide good results in one system may be chemically active in other battery systems. Further, the materials used for the separator are likely to be exposed to highly corrosive liquids present in the cells.
The separator material must not only withstand harsh conditions in the battery cell, but also must be able to withstand fabrication. During cell fabrication, separator materials are subjected to a rolling process which involves a considerable amount of pulling and stretching of the material. For example, in spiral wound cells, strips of anode, cathode and separator material are rolled together in a "jelly roll" fashion and then inserted in a metal can which is then filled with an appropriate electrolyte. Damage to the material during this operation can result in an open structure that may produce shorting in the cell.
Thus, it is desirable to utilize a material which has both excellent mechanical properties and excellent resistance to chemicals present in the battery cell. The combination of these properties is necessary to provide a battery which will have an extended life. However, the materials which meet the above criteria are limited.
In particular, there are very few materials which can be employed as separator materials in lithium/thionyl chloride battery systems due to the inherently harsh environment present within the cells. Lithium is an extremely reactive metal, and materials which may be suitable for use as separators in many battery systems cannot be used in systems with lithium. Further, even if a material is found which does not react with lithium, the material may react with the thionyl chloride (SOCl.sub.2) electrolyte, and would thus not be suitable.
Also, degradation of many common separator materials, such as polyethylene, excludes their use in SOCl.sub.2 electrolytes. Degradation of the binder in an otherwise chemically-resistant separator may also eliminate it for use in such an aqueous environment. For example, U.S. Pat. No. 4,405,700 to Rampel discloses a diacetone acrylamide as a coating for a separator and U.S. Pat. No. 3,904,437 to Specker discloses a polyacrylamide as a component of a battery separator. However, such materials are not suited for lithium/thionyl chloride systems.
Further, abnormal uses (e.g., high rate, over discharge, charge) of the lithium/thionyl chloride cells may cause high temperature excursions, causing the lithium within the cell to be in a molten state. This not only increases the likelihood of chemical reactions and the possibility of fire, but also presents the problem of the molten lithium migrating to the cathode. Even in reserve configurations, degradation may occur by reaction with lithium.
U.S. Pat. No. 4,629,666 to Schlaikjer discloses separators which are suitable for electrochemical cells which contain reactive metals such as lithium and strong oxidants. The separator is a substantially continuous microporous film comprising a polymer of ethylene and a fully halogenated analogue of ethylene (e.g., Tefzel). However, Tefzel is limited in the temperatures in which it retains its mechanical integrity.
Many ceramics are known to be thermodynamically or kinetically stable with lithium. However, these materials possess poor mechanical integrity or impurities which render them less attractive. Ceramic separators have been proposed by Goebel et al in U.S. Pat. No. 4,283,469, Catanzarite in U.S. Pat. No. 4,407,910 and Doddapaneni et al in U.S. Pat. No. 4,598,029.